For example, a technique described in Patent Document 1 below is known regarding such planetary gear mechanisms. In the technique of Patent Document 1, a carrier coupling portion extends radially outside pinion gears so as to be coupled to a fourth coupling member, and extends radially inside the pinion gears so as to be coupled to be coupled to a third coupling member. The carrier coupling portion has columnar insertion holes for the pinion gears to be inserted therethrough. With the pinion gears being inserted through the insertion holes, both axial ends of the pinion gears are supported by a carrier.